flimsy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Light, thin, and insubstantial.
• adjective Lacking solidity or strength; easily damaged: synonym: fragile.
• adjective Lacking plausibility; unconvincing.
• noun Thin paper usually used to make multiple copies.
• noun Something written on this thin paper.

from The Century Dictionary.

• Without material strength or solid substance; of loose and unsubstantial structure.
• Without strength or force of any kind; weak; ineffectual: as, a flimsy argument.
• Synonyms Unsubstantial, thin, slight.
• Feeble, trivial, shallow, superficial, frivolous, foolish, puerile.
• noun A thin sort of paper by means of which several copies of a writing may be made at once; transfer-paper.
• noun A bank-note, from its being made of thin paper.
• noun A manifold copy of a report or despatch made on flimsy; reporters' ‘copy’; a duplicate or a triplicate writing made on flimsy.

from the GNU version of the Collaborative International Dictionary of English.

• noun Thin or transfer paper.
• noun Slang, Eng. A bank note.
• adjective Weak; feeble; limp; slight; vain; without strength or solidity; of loose and unsubstantial structure; without reason or plausibility.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Likely to bend or break under pressure; weak, shaky, flexible, or fragile.
• noun Thin typing paper used to make multiple copies.
• noun informal, in the plural Skimpy underwear.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective lacking solidity or strength
• noun a thin strong lightweight translucent paper used especially for making carbon copies
• adjective lacking substance or significance
• adjective not convincing